Why Do Sharks Expose Their Dorsal

Fins Above the Surface?

Since sharks have gills, they do not need to go to the
surface to breathe. Why do they go to the surface and expose their dorsal fins
outside of the water?

Nicolas
Rimouski, Quebec

Great question, Nicolas!

In movies, sharks are often portrayed with their dorsal fins 'knifing'
through the water. It makes for a dramatic image — the fins look sharp and kind
of dangerous — and one that is easy to fake — it is far cheaper to
build a convincing-looking fin than a realistic-looking whole shark (largely
because — thanks do wonderful underwater documentaries like those on
Discovery's "Shark Week" — most of us know how real sharks
look and move). So, whenever movie-makers want to add a little 'danger' to an
ocean-based story, all they have to do was show one or more phony shark fins
above the surface.

But — in truth — sharks in the wild very, very rarely swim with their
dorsal fins exposed above the surface. Most shark species swim fairly close to
the bottom or in mid-water (the large expanse of ocean between the bottom and
the surface). Left to its own devices, however, most sharks will not approach
the surface at all. Sharks can be lured to the surface with floating bait and,
in investigating such hand-outs, sometimes their dorsal fins break the surface
of the water. Sometimes sharks enter water so shallow that they can barely swim,
and — as a result — their dorsal fins sometimes poke through the surface. Many
species of sharks, for instance, use shallow-water coastal areas as nurseries
for their young (called "pups"); such places offer the newborn sharks
plenty of food but relative safety from fish that are big enough to eat the
pups. On the Queensland coast of Australia, I have seen groups of up to 48
newborn Blackfin Reef Sharks (Carcharhinus melanopterus) swimming in
mangrove swamp waters so shallow that — from a distance — their fins sticking
up through the water looked like a flotilla of tiny sailboats!

It is also not uncommon for largish sharks — such as the Tiger Shark (Galeocerdo
cuvier) — to pursue prey such as schooling fishes and sea turtles into
water so shallow that their dorsal fins, and sometimes their entire backs, are
exposed above the water. Once, on a shallow part of the Great Barrier Reef of
Australia, I saw a small (1-metre or 3-foot long) shark actually 'walk'
completely out of the water. The shark was an Epaulette Shark (Hemiscyllium
ocellatum) — a species named for the large, dark spot on its 'shoulder'.
These prettily marked, slender sharks have highly mobile paired fins —
pectorals behind the gills and pelvics behind the belly — which they use to
clamber over the bottom, looking like a salamander with ping pong paddles for
feet! Anyway, on one particular morning, I saw a little Epaulette Shark actually
crawl out of the ocean, clamber clumsily over some 15 metres (50 feet) of
exposed coral rock, and plunk into a pool of water that had been trapped by the
out-going tide. Unfortunately for the little shark, a Spotted Snake Eel (Myrichthys
maculosus — a small type of eel closely related to the undeservedly dreaded
moray eel) was trapped inside the same tide pool, and wasn't at all happy about
the little shark showing up uninvited. The snake eel wrapped its body around the
Epaulette Shark, and the two writhed and spun in the shallow pool like
play-fighting puppies. After a few seconds of this rough treatment, the little
Epaulette Shark managed to disentangle itself from the eel and clambered out of
the pool and back over the coral rock to the sea.

There may be a very good reason why sharks usually do not expose their dorsal
fins or backs above the water. An American shark biologist named Chris Lowe and
his wife, Gwen Goodman-Lowe, discovered recently that at least some sharks —
like people — actually sun-tan. A part of the sun's light called 'ultraviolet
radiation' can actually penetrate the water up to a distance of about a metre
(three feet) with enough energy that it can cause skin damage to a human or
shark; in response, the skin produces a dark pigment called melanin. Chris and
Gwen demonstrated that young Scalloped Hammerheads (Sphyrna lewini) held
in shallow-water outdoor pools actually tan in a few days. When Chris and Gwen
covered part of the little hammerheads' skin with an opaque (non-see-through)
patch, the only part of the shark that did not tan was that under the patch! I
have seen a similar phenomenon with Nurse Sharks (Ginglymostoma cirratum)
in the Florida Keys: when the sharks first move into shallow water in
preparation for mating, they are a reddish or tan color; but after a few days,
many of them have become a rich chocolate brown. At the time, I thought that
perhaps the sharks were tanning, but no one had ever reported tanning in shark.
Now, thanks to the work of Chris and Gwen Lowe, we know that it is possible.

Unless they, like this Bull Shark
(Carcharhinus leucas) photographed in the Bahamas,
enter extremely shallow water, most sharks do not swim with their dorsal
fins projecting above the surface.

Thus, if sharks can sun tan, it seems likely that they — like humans — can
also sun burn. A dorsal fin poking out of the water for extended periods would
receive much harsher ultraviolet radiation than one that is submerged. Sun
burning could take place in a matter of hours, possibly resulting in a very cranky
shark. So maybe it's a good thing for us that sharks usually don't go swimming
about with their dorsal fins sticking out of the water!